I think the problem is that it's really hard to police recreational quantities of chemicals. Physical evidence related to crimes is usually either bulkier objects which you can't just put down the toilet, or documents which do take some time to destroy and often exist in multiple places.

A sane person would probably argue that you don't get to run roughshod over people's freedoms just because a particular law is hard to enforce otherwise. But I think that's where it's coming from. You get a pervasive environment where the guys at the top are trying to be tough on drugs and the guys at the bottom keep having to let criminals go because they flushed the evidence, and it's only natural that they'll start to push for more draconian measures.